Intermediate · ABOUT 2 MINUTES READING
How Stars Form
A star begins as part of a cold cloud, but becoming a stable star requires gravity, cooling, angular momentum and ultimately nuclear fusion.
Gravity gathers material
Dense regions of molecular clouds can collapse when gravity overcomes supporting effects such as thermal pressure. Collapse is not automatic everywhere in a cloud: turbulence, magnetic fields and external disturbances also matter. As a fragment contracts, gravitational energy heats the interior. Radiation can remove energy and allow contraction to continue.
Disks and outflows
Small initial rotation becomes important during collapse because angular momentum is conserved. Material often forms a disk around the growing protostar rather than falling directly inward. Jets and winds can remove some material and angular momentum. A star-forming region therefore contains both inflow and outflow; not all cloud material becomes part of the star.
Fusion changes the balance
A protostar initially shines mainly from contraction and accretion. Sustained hydrogen fusion eventually provides the central energy source of a main-sequence star if sufficient mass accumulates. Objects below the necessary mass do not sustain ordinary core hydrogen fusion. Dust obscures visible light, making infrared observations especially useful for investigating young stars.
Key takeaways
- Collapse releases gravitational energy.
- Disks and outflows redistribute matter and angular momentum.
- Protostellar luminosity precedes sustained hydrogen fusion.
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